1. Field of the Invention
This invention relates to a motor having a resolver for detecting a position, velocity or acceleration that is determined by the motor and in a specific application, to such a motor associated with a control system that uses the position, velocity or acceleration in controlling the motor.
2. Description of the Related Art
Motors are used in a variety of applications for effecting the controlled motion of objects. For example, motors are used in a variety of industrial automation and other automation applications. In many applications, it is useful to provide the motor or the motor's controller with an accurate measure of the motor position to allow for greater precision in positioning objects with the motor. In other applications, it is useful to provide the motor or its controller with an accurate measure of the velocity or acceleration of the motor's rotor. Position, velocity and acceleration information can be useful however a motor is used, but one or more of these measures may be necessary when the motor is used in closed loop applications. In closed loop systems, one or more sensors collect position, velocity or acceleration information about a motor and provide that information to the motor controller. A closed-loop control system within the motor controller receives the motor position or other information as feedback and improves the accuracy of the positioning or movement characteristics of the motor. Motors whose operation is affected by feedback and under closed loop control are often referenced as servomotors.
To meet these needs, motors are often fitted with sensors that detect the position, velocity or acceleration of the motor. In most applications, only the motor position need be sensed as a function of time and other desired motor positioning characteristics can be derived from the relationship between the motor position and time. Two types of motor position sensors are likely the most prevalent in motors, encoders and resolvers. Encoders provide the motor with an indicator of motor position and a detector that detects changes in the positional relationship between the indicator and the detector. Indicators and encoders vary, but generally rigidly mount the indicator with respect to the rotor or shaft of the motor so that the indicator changes positions as the rotor and shaft change position. The detector is generally mounted in a fixed position, for example on the motor housing, and generally detects changes in the indicator position in a non-contact manner.
Encoder systems include those that use magnetic indicators and sensors and those that use capacitively coupled indicators and sensors. Presently prevalent systems use optical encoders to provide information about the change in motor position. Optical encoders typically affix a glass or other wheel to the motor rotor or shaft that has calibrated markings around the periphery of the wheel. The optical encoder further includes an optical sensor having a light source and a photodiode, with the light source and the photodiode positioned on opposite sides of the wheel so that rotation of the wheel causes the markings on the wheel to modulate the light path between the source and the photodiode. This modulation is detected and used to detect changes in the motor's position. Such an encoder can be used in the closed loop control of the motor and can be used to provide motor position information.
Encoders can provide accurate positioning and control information. On the other hand, encoders can be an expensive component in that an encoder can be a major portion of the costs of a motor. Encoders require alignment and additional wiring and can add significantly to the assembly costs of a motor. Encoders also typically have a temperature range that is smaller than the temperature range of the motor to which they are attached, limiting the use of the motor to the smaller temperature range of the attached encoder. This can limit the power that can be practically achieved with the system due to the reduced temperature range. It would be desirable, for at least some motor applications, to provide a simpler or more cost effective strategy for measuring motor position or other motor characteristics.
Resolvers represent a different strategy for measuring the position and other movement characteristics of motors. Resolvers can be viewed as rotary transformers and generally have structures similar to motors. That is, resolvers include a rotating or otherwise moving rotor and a stationary stator. One or more coils are generally provided on the rotor and the stator, although this is not the only configuration known. The resolver rotor is attached to a shaft and generally one or more of the windings of the stator or rotor are driven with an alternating signal. Signals from the undriven coils are extracted and processed to yield position or velocity information about the shaft to which the rotor is coupled. As a general matter, resolvers are added to motors as distinct structures and so are not integrated with the electronics or magnetics of the motor. When a resolver is implemented as an add on to an existing motor structure, it increases the cost of the components of the motor and also increases the assembly costs for the motor.
There have been attempts to integrate a motor with a resolver. For example, U.S. Pat. No. 4,980,594 shows a combination servomotor and resolver. The design described in this patent is a complicated one that can be difficult to assemble. Windings are added to the stator and the rotor of the illustrated servomotor. A rotary transformer couples an excitation signal to the resolver coils on the rotor; that signal is detected and read from the resolver coils of the stator. Because drive signals need to be provided to the rotor, it is necessary to provide an inductive coupling, a slip ring coupling or other technique for coupling the signals to the rotating body. Generally such contacts are difficult to make and can be unreliable. The driving signal applied to the rotor is generated by additional electronics that are not normally used or present in the motor drive electronics.
Aspects of the present invention are discussed in terms of a synchronous, high pole count motor. Such motors are sometimes referenced as stepper motors and can be operated either in an open loop configuration or in a closed loop configuration such the stepper motor can also be operated as a servomotor. High pole count synchronous motors have certain properties that are used advantageously in some implementations of the present invention. U.S. Pat. No. 4,025,810 entitled “Low Noise Synchronous Motors” describes basic aspects of the configuration and operation of a stepper motor that are useful to this discussion.
FIG. 1 shows a view of a rotary stepper motor in cross section that is simplified from that described in U.S. Pat. No. 4,025,810. Referring to FIG. 1, the motor 10 has a central shaft 12 extending through a rotor 14. The rotor 14 includes a central permanent magnet 16, 18 magnetized in a vertical direction as shown in FIG. 1. For discussion purposes, the upper portion 16 of the permanent magnet can be taken as the north pole and the lower portion 18 can be taken as the south pole. Pole caps or pole pieces 20, 22 are positioned over the ends of the central permanent magnet 16, 18, respectively. The pole caps 20, 22 are made of a high permeability material through which the magnetic field of the permanent magnet 16, 18 passes readily. The magnetic field from the permanent magnet couples through the respective north and south pole caps 20, 22 and through the stator that encircles the rotor 14. FIG. 1 schematically shows that there are teeth 26 on the outer periphery of the rotor cap piece 20 and teeth 28 on the inner periphery of the pole pieces of the stator 24. These teeth are somewhat like the teeth of a gear and are designed so that they can come into close alignment to provide low reluctance paths between the rotor and the stator.
FIG. 2 shows one cross-section through the upper portion of the FIG. 1 motor; FIG. 2 shows the teeth of the pole caps and the stator more clearly. Like structures are identified in FIG. 2 with the numerals introduced in FIG. 1 for ease in understanding the discussion. Thus, FIG. 2 shows in cross section the shaft 12 surrounded by the north pole portion 16 of the permanent magnet. Pole cap 20 is shown in the cross section of FIG. 2 as a ring around the permanent magnet, with teeth 26 arranged on the outer cylindrical surface of the pole cap. Because the pole cap is a high permeability material, the teeth 26 and the other teeth extending from the pole cap act as north poles of a permanent magnet. Stator 24 surrounds the rotor, with a continuous outer ring and eight pole pieces extending inward from the outer ring. Teeth 28 extend inwardly from the ends of the stator pole pieces. The pole pieces are wrapped with coils that can be energized to produce magnetic fields at the teeth 28. The coils can be wound around the stator pole pieces in different directions, depending on the number of phases used in the motor.
At the position 30 indicated in FIG. 2 (i.e., the nine o'clock position), the teeth 26 of the pole cap align with the teeth 28 on the pole piece of the stator. This is a minimum reluctance path between the rotor and the stator for magnetic fields. At the position 32 (i.e., the six o'clock position), the teeth of the rotor cap piece and the stator pole piece are maximally (½ tooth) out of alignment. This is a locally maximum reluctance path between the rotor and the stator.
FIG. 3 shows another view through the motor of FIG. 1, with most of the cross section of FIG. 2 shown, but with a cut away to show in part a cross section through the lower portion of the motor. In that cut out portion, this cross section showing the outer portion of the lower, south pole rotor cap 22. As shown in FIG. 1, stator pole pieces extend the height of the motor while the rotor cap pieces are separated. Thus the stator teeth extend continuously vertically along the inner periphery of the stator. The rotor pole caps are separate pieces and the teeth on the pole caps 20, 22 are misaligned with respect to each other by one half tooth. In other words, the south pole cap 22 is rotated by one half tooth with respect to the north pole cap. The consequence of this alignment is that, at the position 34 shown in FIG. 3, which is vertically aligned with the nine o'clock position 30 shown in FIG. 2, the teeth 36 on the south pole cap are maximally misaligned with the teeth 28 of the stator pole piece. Thus, while at this particular rotation this stator pole piece has a minimum reluctance path to the north pole rotor cap 20, the stator pole piece has a locally maximum reluctance path to the south pole cap 22.